Presently, a broadband ISDN, a broadband LAN and the like are expected as communication networks of the next generation which provide a broadband service such as image communication and high-speed data communication. In such a broadband service, the needs of the service are uncertain. Hence, an ATM (asynchronous transfer mode) has been proposed to transfer packets (cells) of fixed length at a high speed, in order to uniformly and efficiently transfer and switch the various kinds of services related to audio, video and data. The cell in this case is transferred in a burst form only when an information transfer request is generated, thereby enabling a communication which is not dependent on the speed. In addition, when multiplexing the data in units of cells as in the ATM, the competitive access becomes a problem because a plurality of sub stations within the communication network make access to a line.
For the above described reasons, there is a demand for a simple multiple access system which is efficient and does not introduce the competitive access.
One example of the conventional multiple access system is the request assign system employed in the LAN or the like. According to this request assign system, the sub station which has a transmission request makes the transmission request by adding a request flag on an up-line. The main station detects the request flags of all the sub stations, and assigns the authority to transmit according to a predetermined rule (for example, in the sequence in which the requests are detected). Information related to the authority to transmit is notified by a transmission enable flag on a down-line, and the sub station transmits data according to the content of this flag.
FIG. 1 shows an example of a data format in such a conventional system, FIG. 2 shows an example of a construction of the sub station, and FIG. 3 shows an example of the construction of the main station.
First, as shown in FIG. 1(B), request information REQ made up of bit regions R1 through Rn respectively corresponding to n terminals is added to an up-line data, and data transmission enable information AVA is added to a down-line data as shown in FIG. 1(A). In addition, DL indicates a beginning of the data format.
When such a data format is used and a transmission data DATA-TE amounting to a predetermined transmission unit is stored in a data buffer 11 of the sub station shown in FIG. 2, a request flag REQ becomes "1". A multiplexer part MUX multiplexes the request flag REQ to the transmission data DATA-TE and transmits it to the main station.
In this case, the request flag REQ is provided for each sub station, and a sub station i, for example, uses a request flag Ri as shown in FIG. 1(B).
At the main station, a demultiplexer part DMUX demultiplexes the request flags REQ from all of the sub stations as shown in FIG. 3, and a sub station number of the request flag REQ which is "1" is stored in a memory 12. The sub station numbers stored in the memory 12 are read as as the data transmission enable information AVA according to a predetermined rule (for example, in the sequence with which the numbers are stored), and is transmitted to the sub station by multiplexing it on the down-line in a multiplexer part MUX.
In the sub station shown in FIG. 2, the data transmission enable information AVA which is demultiplexed in a demultiplexer DMUX is read, and if this information AVA matches its own sub station number, the data buffer 11 is opened to transmit the information AVA to the main station by multiplexing it on the up-line in the multiplexer part MUX.
By carrying out the above described operation, the competitive access of the line from the plurality of sub stations is eliminated.
According to the conventional request assign system described above, there is a waiting time from a time when the data amounting to the predetermined transmission unit is stored in the data buffer 11 and the transmission request information is transmitted to a time when the authority to transmit data is assigned by the main station. Such a delay time becomes a problem in CBR (constant bit rate) services such as audio and video services.